While the present invention was developed for use in aircraft, and is described in an aircraft environment, it is to be understood that the invention can be utilized to create honeycomb core structures with embedded fasteners for use in other environments.
In recent years, more and more attention has been directed to creating new and better aircraft parts from various types of high-strength, lightweight materials that were not previously used in aircraft. The materials range from new metal alloys, such as new titanium and aluminum-lithium allosy, to a variety of synthetic materials, such as graphite/epoxy composites. The parts range from structural components, such as stringers, frame elements and gussets, to aerodynamic elements, such as ailerons, trim tabs and landing gear doors. In some instances, proposals have been made to create parts by joining together pieces made of different materials. One such proposal is to create parts formed of sandwich structures comprising honeycomb core layers and skin layers bonded together, the skin layers being formed of nonmetallic synthetic materials, such as graphite/epoxy composites. One difficulty associated with such structures, which are herein referred to generically as synthetic skin/honeycomb core structures, has been installing fasteners in a manner that results in the fasteners having adequate load carrying capability, In the past, fasteners have been installed in synthetic skin/honeycomb core structures by drilling a hole in the structures after they have been formed and installing truss head fasteners in the holes. The large underside surface of truss head fasteners distributes head force over a relatively large area of the synthetic skin.
While truss head fasteners have adequate load capability, they have a number of disadvantages when used with synthetic skin/honeycomb core structures. Specifically, because truss head fasteners create pimple-like protrusions of the surface of the skin against which the heads impinge, the skin surface is not aerodynamically smooth. As a result, such skin surfaces are less than ideal for parts used to form the wings, fuselage and empennage of an aircraft, such as ailerons, tabs, landing gear doors, etc. Further, because most fasteners are formed of metal, which is electrically conductive, truss head fasteners provide an electrical conducting path between the nonmetallic surface against which the truss head fasteners impinge and internal metallic components. This is particularly undesirable when the part is an external part, such as a tab, aileron or landing gear door, because the truss head fastener provides an electrical conducting path for lightening striking the part. Further, because truss head fasteners require a hole through the skin layers of synthetic skin/honeycomb core parts, they create potential moisture paths, particularly when fasteners loosen, which can cause corrosion of the core, if the core is formed of a material that is subject to corrosion.
The invention is directed to providing a method of embedding a fastener in synthetic skin/honeycomb core structures that avoids the foregoing disadvantages. More specifically, the invention is directed to providing a method of embedding a fastener in a synthetic skin/honeycomb core structure that has adequate load carrying capability and leaves the other surface of at least one skin unbroken and aerodynamically smooth.